1. Field of the Invention
The present invention relates to the field of thyristors and triacs and more particularly to lateral, metal-oxide-semiconductor (MOS) controlled thyristors and triacs.
2. Prior Art
A thyristor is a semiconductor device having four layers (NPNP) which (1) can conduct current in only one polarity, (2) is capable of blocking voltage applied across its main electrodes in either of two opposing polarities, (3) has an ON state in which it can be latched and (4) can be triggered into that ON state by any one of a variety of techniques when a voltage of appropriate polarity and magnitude is applied across its main electrodes.
A triac is a five layer device which is essentially two thyristors constructed in the same semiconductor material in antiparallel relation. A triac can be triggered into its ON state to conduct in either direction, but only one of its component thyristors conducts at a time. Which component thyristor of a triac conducts is determined by the polarity of the voltage applied across the main terminals. When one of the antiparallel thyristors is conducting at the time the bias across the triac's main terminals is reversed, the stored carriers in the conducting triac must be prevented from triggering the other thyristor ON, or else the triac will remain in an ON state despite the reversal in the bias across the main electrodes. Such triggering can occur when the two thyristors are close enough together within the semiconductor body that the charges stored in the conducting thyristor have spread or can spread into the other thyristor in sufficient quantity to trigger it ON upon reversal of the main bias.
Some thyristors, once they have been triggered ON, remain ON until the bias across their main electrodes is removed or reversed. Other thyristors can be turned OFF by an appropriate signal applied to a control gate even while a bias encouraging continued conduction is maintained across their main electrodes. Such thyristors are known as gate turn-off thyristors. Triacs of both types are also known.
Thyristors and triacs are primarily constructed with their main current path running through the thickness of a semiconductor wafer between opposed major surfaces. Such thyristors are known as vertical current or vertical thyristors. One disadvantage of such vertical current devices is the fact that patterned device regions and electrodes may have to be provided on both surfaces of the semiconductor wafer. This complicates device fabrication and makes mounting the device in a package more difficult than is the case with devices which have one side which is uniform with a single electrode thereon.
Lateral structures in which all of the device patterning is done on a single surface of the semiconductor device are desirable from the fabrication and device connection points of view because the surface of the device opposite the patterned surface can be uniform or unpatterned and can be easily attached to a mounting surface in a package.
Lateral thyristors and triacs have been developed, however their operating characteristics, such as ON-resistance and current density, are generally inferior to those of vertical devices and none are available which provide gate turn-off of high current densities such as 100 amperes per square centimeter. In order for lateral triacs to provide maximum utility, they must be responsive to gate control both for turn-on and turn-off and must have the capacity to carry high currents and to turn those high currents off under gate control. Metal-oxide-semiconductor (MOS) gate control of thyristor and triac conduction is desirable because small amplitude control currents (essentially voltage control) can then control large amplitude currents in the main current path of the device.
An object of this invention is to provide lateral triacs capable of handling substantially increased current densities while providing gate control of both device turn on and device turn-off.
A further object is to provide lateral triac structures which provide device current density and voltage drop characteristics which are comparable to those of vertical devices.